Option to dynamically use Master-Page for Page Include reasons

In trying to keep page load times as fast as possible while also leveraging existing pages without having to build pages, I would like to use Child Pages without any of the master-page elements. 

My use case is to use something like child Opportunities or Contact Tab pages as a subtab to Account. Same deal with Tasks and Events.

Did you try master pages where the only component in page region is a page include. Then you can create a “with master” and “without master” version of each page

That would defeat the purpose though. I’d like the child page to load as is at times without a page include. Load time is optimal this way.